A generic seat belt retractor is known for example from DE 199 27 427 C2. The seat belt retractor described by that reference has a two-stage load limiting system in which the load limitation level can be switched from a high load limitation level to a low load limitation level. The load limiting device further includes a torque tube between the belt shaft and the load limiting device designed as a torsion bar, which torque tube is releasably locked in relation to the belt shaft. When the torque tube is released, the load limitation level is switched from the high level to the low level. In order that there is a smooth transition between the high level and the low level, an additional deformable element shall be located between the torque tube and the belt shaft. The above-mentioned publication does not give any information about the design of this deformable element.
From DE 10 2007 057 661 A1, a self-locking seat belt retractor having the same configuration is known in which the third load limiting element is formed by a bending wire located parallel to the belt shaft. The contribution of the third load limiting element to the load limitation characteristic is effected by the bending wire being wound around the axis of a torsion bar at the same time being plastically deformed out of the parallel alignment to the belt shaft. Thus, an installation space needs to be provided in the seat belt retractor suitable for mounting the bending wire prior to deformation as well as after deformation. Furthermore, the load limitation level to be realized by the third load limiting element is limited by the plastic deformation energy of the bending wire and can only be increased by enlarging the bending wire itself. As, according to today's requirements of the vehicle manufacturers, seat belt retractors shall be designed to be as small as possible, it might be difficult for installation space reasons to achieve the required load limitation characteristic with this solution.
Therefore, it is the object of the invention to provide a self-locking seat belt retractor comprising a two-stage load limiting device with two load limiting elements having different load limitation levels and a third load limiting element acting parallel to the load limiting element having the low load limitation level, which seat belt retractor shall take up as little installation space as possible.
For the solution of the object, the invention proposes a self-locking seat belt retractor comprising the features of claim 1.
The basic idea of the invention is that the third load limiting element is formed by at least two structural elements resting against each other within the force flow between the locking device and the belt shaft, at least one of which structural elements is sheared off during the load limitation.
With the proposed solution, the energy dissipation upon the activation of the third load limiting element is effected by shearing off the structural elements. The shearing-off process is a deformation effected by shear forces allowing a maximum energy dissipation in a minimum space provided the structural elements have an appropriate shape and the material of the structural elements is chosen accordingly. As the deformation of the structural elements subject to the shear forces, besides the plastic deformation, generally implies the destruction of the structural elements, more energy can be consumed thereby than it would be possible with a deformation subject to bending forces. Furthermore, the structural elements are not moved from one installation space to another installation space during the load limitation so that the proposed load limiting element in itself can be regarded as being stationary requiring less installation space. Furthermore, the load limitation effected by the third load limiting element, due to the structural elements resting against each other, starts immediately when the relative rotational movement is initiated so that no installation space is wasted either thereby.